Different surface features exhibit different scattering characteristics:
urban areas: very strong backscatter
forest: intermediate backscatter
calm water: smooth surface, low backscatter
rough sea: increased backscatter due to wind and current effects
The radar backscattering coefficient σ 0 provides information about the imaged surface. It is a function of:
- radar observation parameters:
(frequency f, polarisation p and incidence angle of the electromagnetic waves emitted);
(roughness, geometric shape and dielectric properties of the target).
Influence of frequency
The frequency of the incident radiation determines:
- the penetrationdepth of the waves for the target imaged;
- the relative roughness of the surface considered.
Penetration depth tends to be longer with longer wavelengths. If we consider the example of a forest, the radiation will only penetrate the first leaves on top of the trees if using the X-band (λ = 3 cm). The information content of the image is related to the top layer and the crown of the trees. On the other hand, in the case of L-band (λ = 23 cm), the radiation penetrates leaves and small branches; the information content of the image is then related to branches and eventually tree trunks.
The same phenomenon applies to various types of surfaces or targets (see the figure).
But it should be noted that:
- penetration depth is also related to the moisture of the target;
- microwaves do not penetrate water more than a few millimeters.
Influence of polarization
Polarization describes the orientation of the electric field component of an electromagnetic wave. Imaging radars can have different polarization configurations.
However, linear polarization configurations HH, VV, HV, VH are more commonly used. The first term corresponds to the polarization of the emitted radiation, the second term to the received radiation, so that XHV refers to X band, H transmit, and V receive for example.
In certain specific cases, polarization can provide information on different layers of the target, for example flooded vegetation. The penetration depth of the radar wave varies with the polarization chosen.
Polarization may provide information on the form and the orientation of small scattering elements that compose the surface or target.
More than one bounce of backscattering tends to depolarize the pulse, so that the cross polarized return in this case would be larger than with single bounce reflection.
Influence of roughness
Roughness is a relative concept depending upon wavelength and incidence angle.
A surface is considered "rough" if its surface structure has dimensions that are comparable to the incident wavelength.
According to the Rayleigh criterion, a surface is considered smooth if:
and considered rough if:
An example of the effect of surface roughness can be observed in the zones of contact between land and water.
Inland water bodies tend to be relatively smooth, with most of the energy being reflected away from the radar and only a slight backscatter towards the radar.
On the contrary, land surfaces tend to have a higher roughness.
Water bodies generally have a dark tonality on radar images, except in the case of wind-stress or current that increase the water surface roughness, which
provokes a high backscatter (see Bragg scattering).
In the microwave region, this difference between respective properties of land and water can be extremely useful for such applications as flood extent measurement or coastal zones erosion. This animation illustrates the range of backscatter from water surfaces.
Influence of incidence angle
The incidence angle is defined by the angle between the perpendicular to the imaged surface and the direction of the incident radiation. For most natural targets, backscatter coefficient s 0 varies with the incidence angle.
Experimental work was conducted by Ulaby et al. (1978) using five soils with different surface roughness conditions but with similar moisture content. It appeared that, when using the L band (1.1 GHz), the backscatter of smooth fields was very sensitive to near nadir incidence angles; on the other hand, in the case of rough fields, the backscatter was almost independent of the incidence angle chosen.
Influence of moisture
The complex dielectric constant is a measure of the electric properties of surface materials. It consists of two parts (permittivity and conductivity) that are both highly dependent on the moisture content of the material considered.
In the microwave region, most natural materials have a dielectric constant between 3 and 8, in dry conditions. Water has a high dielectric constant (80), at least 10 times higher than for dry soil.
As a result, a change in moisture content generally provokes a significant change in the dielectric properties of natural materials; increasing moisture is associated with an increased radar reflectivity.
The electromagnetic wave penetration in an object is an inverse function of water content. In the specific case of vegetation, penetration depth depends on moisture, density and geometric structure of the plants (leaves, branches).